lecture notes

1. Lecture 1: Introduction to
Operating Systems
Lecture 1: Introduction to
Operating Systems
Mythili Vutukuru
IIT Bombay

2. What is an operating system?
• Middleware between
user programs and
system hardware
• Manages hardware:
CPU, main memory, IO
devices (disk, network
card, mouse,
keyboard etc.)
• Middleware between
user programs and
system hardware
• Manages hardware:
CPU, main memory, IO
devices (disk, network
card, mouse,
keyboard etc.)
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3. What happens when you run a
program? (Background)
• A compiler translates high level programs into an
executable (“.c” to “a.out”)
• The exe contains instructions that the CPU can
understand, and data of the program (all
numbered with addresses)
• Instructions run on CPU: hardware implements
an instruction set architecture (ISA)
• CPU also consists of a few registers, e.g.,
– Pointer to current instruction (program counter or PC)
– Operands of instructions, memory addresses
• A compiler translates high level programs into an
executable (“.c” to “a.out”)
• The exe contains instructions that the CPU can
understand, and data of the program (all
numbered with addresses)
• Instructions run on CPU: hardware implements
an instruction set architecture (ISA)
• CPU also consists of a few registers, e.g.,
– Pointer to current instruction (program counter or PC)
– Operands of instructions, memory addresses 3

4. So, what happens when you run a
program?
• To run an exe, CPU
– fetches instruction pointed at
by PC from memory
– loads data required by the
instructions into registers
– decodes and executes the
instruction
– stores results to memory
• Most recently used
instructions and data are in
CPU caches for faster access
• To run an exe, CPU
– fetches instruction pointed at
by PC from memory
– loads data required by the
instructions into registers
– decodes and executes the
instruction
– stores results to memory
• Most recently used
instructions and data are in
CPU caches for faster access
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5. So, what does the OS do?
• OS manages program
memory
– Loads program executable
(code, data) from disk to
memory
• OS manages CPU
– Initializes program
counter (PC) and other
registers to begin
execution
• OS manages external
devices
– Read/write files from disk.
• OS manages program
memory
– Loads program executable
(code, data) from disk to
memory
• OS manages CPU
– Initializes program
counter (PC) and other
registers to begin
execution
• OS manages external
devices
– Read/write files from disk.
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6. OS manages CPU
• OS provides the process abstraction
– Process: a running program
– OS creates and manages processes
• Each process has the illusion of
having the complete CPU, i.e., OS
virtualizes CPU
• Timeshares CPU between processes
• Enables coordination between
processes
• OS provides the process abstraction
– Process: a running program
– OS creates and manages processes
• Each process has the illusion of
having the complete CPU, i.e., OS
virtualizes CPU
• Timeshares CPU between processes
• Enables coordination between
processes
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7. OS manages memory
• OS manages the memory
of the process: code, data,
stack, heap etc
• Each process thinks it has a
dedicated memory space
for itself, numbers code
and data starting from 0
(virtual addresses)
• OS abstracts out the details
of the actual placement in
memory, translates from
virtual addresses to actual
physical addresses
• OS manages the memory
of the process: code, data,
stack, heap etc
• Each process thinks it has a
dedicated memory space
for itself, numbers code
and data starting from 0
(virtual addresses)
• OS abstracts out the details
of the actual placement in
memory, translates from
virtual addresses to actual
physical addresses
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8. OS manages devices
• OS has code to manage disk, network card,
and other external devices: device drivers
• Device driver talks the language of the
hardware devices
– Issues instructions to devices (fetch data from a
file)
– Responds to interrupt events from devices (user
has pressed a key on keyboard)
• Persistent data organized as a filesystem on
disk
• OS has code to manage disk, network card,
and other external devices: device drivers
• Device driver talks the language of the
hardware devices
– Issues instructions to devices (fetch data from a
file)
– Responds to interrupt events from devices (user
has pressed a key on keyboard)
• Persistent data organized as a filesystem on
disk
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9. Design goals of an operating system
• Convenience, abstraction of hardware
resources for user programs
• Efficiency of usage of CPU, memory, etc.
• Isolation between multiple processes
• Convenience, abstraction of hardware
resources for user programs
• Efficiency of usage of CPU, memory, etc.
• Isolation between multiple processes
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10. History of operating systems
• Started out as a library to provide common
functionality across programs
• Later, evolved from procedure call to system
call: what’s the difference?
• When a system call is made to run OS code,
the CPU executes at a higher privilege level
• Evolved from running a single program to
multiple processes concurrently
• Started out as a library to provide common
functionality across programs
• Later, evolved from procedure call to system
call: what’s the difference?
• When a system call is made to run OS code,
the CPU executes at a higher privilege level
• Evolved from running a single program to
multiple processes concurrently
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